The present invention relates generally to gas fired combustion apparatus such as residential and light commercial furnaces and the like. More particularly, the present invention relates to a combustion system for use in such a gas fired apparatus characterized by a reduced level of emission of oxides of nitrogen (NOx).
During the combustion of fossil fuels, including gaseous fuels such as natural gas, liquefied natural gas and propane, for example, in air, NOx is formed and emitted to the atmosphere in the combustion products. With respect to gaseous fuels that contain little or no fuel-bound nitrogen per se, NOx is largely formed as a consequence of oxygen and nitrogen in the air reacting at the high temperatures resulting from the combustion of the fuel.
Governmental agencies have passed legislation regulating the amount of oxides of nitrogen that may be admitted to the atmosphere during the operation of various combustion devices. For example, in certain areas of the United States, regulations limit the permissible emission of NOx from residential furnaces to 40 ng/J (nanograms/Joule) of useful heat generated by these combustion devices. It is expected that future regulations will restrict NOx emissions from residential furnaces and boilers to even lower levels.
Gas fired apparatus, such as residential and light commercial heating furnaces, often use a particular type of gas burner commonly referred to as an in-shot burner. An in-shot burner comprises a burner nozzle having an inlet at one end for receiving separate fuel and primary air streams and an outlet at the other end through which mixed fuel and primary air discharges from the burner nozzle in a generally downstream direction. The burner nozzle may simply comprise an axially elongated, straight tube, or it may comprise a generally tubular member, which may be arcuate or straight, having an inlet section, an outlet section and a transition section, commonly referred to as a venturi section, disposed therebetween. Fuel gas under pressure passes through a central port disposed at or somewhat upstream of the inlet of the burner nozzle. The diameter of the inlet to the burner nozzle is larger than the diameter of the fuel inlet so as to form an annular area through which atmospheric air is drawn into the burner nozzle about the incoming fuel gas. This primary air mixes with the fuel gas as it passes through the tubular section of the burner nozzle to form a primary air/gas mix. This primary air/gas mix discharges from the burner nozzle through the outlet of the burner nozzle and ignites as it exits the nozzle outlet section forming a flame projecting downstream from a flame front located adjacent or somewhat downstream of the outlet of the burner nozzle. Secondary air flows around the outside of the burner nozzle and is entrained in the burning mixture downstream of the nozzle in order to provide additional air to support combustion.
In conventional practice, a flame retention device is often inserted within the outlet section of the burner in an attempt to achieve improved flame stability and reduction of noise. One known insert comprises a cylindrical body defining a central opening and having a toothed perimeter formed by a plurality of circumferentially spaced, axially elongated splines extending radially outwardly in a sunburst pattern about the circumference of the cylindrical body.
U.S. Pat. No. 6,145,501, assigned to the assignee of the present invention, shows an in-shot burner having a catalyst disposed in its outlet end thereof for the purpose of catalyzing the fuel in the primary air/fuel mixture to intermediate combustion species to thereby reduce emissions such as nitrogen oxides. In the example described, the total air provided is 145% of that required for stiochiometric combustion, with primary air being provided at about 50%, thereby reducing NOx to 28.59 ppm, or 22 ng/J. While this may meet the needs for NOx reduction, it will require the catalyst to operate at relatively high temperatures so as to thereby result in a relatively short life (i.e.  less than 1000 hours of operation) of the catalyst.
U.S. Pat. No. 4,776,320, Ripka et al., discloses a gas-fired furnace utilizing an in-shot burner wherein a thermal energy radiator structure, such as a perforated stainless steel structure, is disposed in the flame downstream of the burner outlet. The radiator structure tempers the flame by absorbing heat therefrom and radiating the absorbed heat to the surrounding heat transfer surface, whereby peak flame temperatures are limited and NOx formation is reduced.
A problem associated with the reduction of nitrogen oxide formation by lowering the flame temperature is that as the flame is quenched, combustion may not be totally completed. As a consequence of flame quenching, carbon monoxide formation will increase as nitrogen oxide formation decreases. Thus, the radiator structure of the ""320 patent would be capable of reducing NOx emissions from 45 ng/J to 35 ng/J at acceptable CO levels. Attempts to lower NOx further, however, would result in the generation of carbon monoxide at a level above that permitted by regulations.
To avoid the consequence of increased carbon monoxide formation associated with reduction of NOx emissions by reducing peak flame temperatures, attempts have been made to reduce nitrogen oxides formation by using a catalyst to promote chemical reactions which result in a reduction of NOx formation in the flame. U.S. Pat. No. 5,746,194, Legutko, discloses a combustion system having an in-shot burner wherein a flow dividing member supports a partial oxidation catalyst disposed in the fuel rich inner core of the flame downstream of the burner outlet. The catalyst serves to catalyze unburnt methane in the fuel rich inner core of the flame to hydrogen and carbon monoxide. When this hydrogen and carbon monoxide subsequently combust in the air rich outer zone of the flame, the peak combustion temperatures are lower than in conventional combustion and NOx formation is reduced. The catalytic insert is heated above the reaction xe2x80x9clight-offxe2x80x9d temperature of the catalyst directly by the flame itself. The catalytic insert also radiates heat away from the flame to further reduce peak temperature within the flame. While such an arrangement results in reduced NOx levels, while at the same time limiting the generation of CO, because the catalyst is disposed in the flame, it is difficult to maintain the temperature of the catalyst at a level low enough to ensure long-term reliability thereof.
U.S. Pat. No. 5,848,887, Zabielski et al., shows another approach for using both a catalyst and a radiation body for decreasing NOx while limiting the generation of CO. The radiator body is disposed in the flame downstream of an in-shot burner to quench the flame to reduce NOx formation, while the catalyst is disposed further downstream of the flame in a lower temperature region for oxidizing carbon monoxide in the flue gas to carbon dioxide. In this way, the catalyst is provided to clean up the CO which is generated by the radiating body, and the problem of exposing the catalyst to high temperatures and a short life, is solved by locating the catalyst at a relatively remote location downstream where the temperatures are not excessive. However, in the event that the catalyst does become ineffective for any reason, the resulting system will be similar to that described in the ""501 patent discussed hereinabove wherein the heat radiating device will reduce NOx but may cause excessive levels of CO to be present.
It is therefore an object of the present invention to provide an improved fuel air combustion apparatus and method of operation.
This object and other features and advantages become readily apparent upon reference to the following descriptions when taken in conjunction with the appended drawings.
Briefly, in accordance with one aspect of the invention, a catalyst is provided at a position substantially upstream of the flame, and the amount of primary air which is provided to the burner is limited so as to thereby reduce NOx emissions from the burner but maintain a relatively low temperature at the catalyst and thereby prolong its life. In one embodiment, the catalyst is composed of a ceramic honeycomb material with a noble metal (i.e. rhodium, platinum or palladium), and the amount of primary air is limited to 45 percent of that required for stoichiometric combustion such that the temperature of the catalyst does not exceed 2000 deg. F.
In accordance with another aspect of the invention, a baffle is provided in the flame so as to radiate heat therefrom to further reduce NOx emissions. The mass of the radiation baffle is limited so as not to reduce the flame temperature to a level which will cause any significant generation of CO.
In accordance with yet another aspect of the invention, the amount of primary air being provided to the burner is controlled to at least 25 percent of that required for stoichiometric combustion, such that, in the event of a catalyst failure, complete combustion of the fuel/air mixture will occur.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.